Sigil is believed by some to be located inside of Cynosure, a plane outside of the Realmspace crystal sphere. It has the shape of a torus; the city itself is located on the inner surface of the ring. There is no sky, simply an all-pervasive light that waxes and wanes to create day and night. Sigil cannot be entered or exited save via portals; although this makes it quite safe from any would-be invader, it also makes it a prison of sorts for those not possessing a portal key. Thus, many call Sigil "The Bird Cage" or "The Cage". Though Sigil is commonly held to be located "at the center of the planes" (where it is positioned atop the infinitely tall Spire), some argue that this is impossible since the planes are infinite in all dimensions, and therefore there can never truly be a center to any of them, let alone all of them; thus, Sigil is of no special importance. Curiously, from the Outlands one can see Sigil atop the supposedly infinite Spire.

Sigil contains innumerable portals: any bounded opening (a doorway, an arch, a barrel hoop, a picture frame) could possibly be a portal to another plane, or to another point in Sigil itself. Thus, the city is a paradox: it touches all planes at once, yet ultimately belongs to none; from these characteristics it draws its other name: "The City of Doors."

The ruler of Sigil is the mysterious Lady of Pain. The Lady is sometimes seen in Sigil as a floating, robed Lady with a face bearing a mantle of blades. The Lady does not concern herself with the laws of the city; she typically only interferes when something threatens the stability of Sigil itself. However, she is an entity of inscrutable motives, and often those who cross her path, even accidentally, are flayed to death or teleported to her hidden Mazes, lost forever. It is widely believed that she never speaks, although some unconfirmed (and, most would argue, highly questionable) rumours to the contrary do exist.

Sigil is, theoretically, a completely neutral ground: no wars are waged there and no armies pass through. Furthermore, no deities can enter into Sigil; the Lady has barred them from the Cage. Of course, Sigil is hardly peaceful; with such a condensed population, consisting of everything from angelicdevas to demonicglabrezu, violence is common, usually befalling the foolhardy, the incautious, or the poor. Most natives of Sigil ("Cagers") are quite jaded as a result of living there.

People coming to Sigil from the Prime Material Plane are often treated as clueless inferiors by the planar elitists who dwell there. They are thus widely referred to as the "Clueless", "Berks" or more charitably, as "Primes".